Band pass filter



Nov. 24, 1953 V. D. LAN DON BAND PASS FILTER Filed July 27, 1949 ATTORNEY Patented Nov. 24, 1953 2,560,712.; BnNDfPAss Florine Vernon.. D. Landonl Princeton. Ni .I1-, assigner'.A to Radio Corporation of America?. incorporation.

ofIDelaware Application Jnlye 27; 1949; SerialENo.` 1117,1'1174 (Cl. S33- 71) Claims, 1.

Thisinlvention relatesftoelectrcal lters and;y parti,cularlylv to. band pass lters1 havingf relativer. ly. narrow pass,1 bands@ for use at medium; fre: quencies.

One of. the commenlygu usefinetworks constituting a band pass. lter, is. the? sri-called*- Mr derived. 1t,er.. Essentially.,it consists of` two series.. ations;A or. branches. and prefer-alcnlyv one.A shunt: arnfir or branchy connected.; symmetricallyy to; the? series arms. When it is. desired tofuseza filter, ci such, character at medium frequencies. suonl asthoseofgthe-orden of` kilocycles, it...isinecesL-Ln sary in, order. to povidefairelatively. narrowl pass.` bend. that1 thecircuitelements, of.. the shunt; have relatively high Qs so-that the lter.; may.` have the necessary.. sharp, cutols`- at theI limitine., frequencies ofthe, pass hand Atsuoh frer quenceshoiveven, itis dicult to :construct: coilsfor use intheshunt arm of. arnM-derived lter*- Whieh. will. have.. a sufficiently high. Q. to effen?. the desiredl sharp cutoifg at the. limiting. free. queniea This is p articularlf(l true, when the` ratiov of; themean frequency2 ofi the.. pass. band; te the. band Width isrelatively. high, such., as.; of. the order of 2,0".

Aooordinglm. it, is` en. object of. this; invention.` tolprovide anlimprovedband passilter, for,- use at medium frequencies which. willA have sharp cutoffs at thelimitinglfrequencies in cases where.V

it is desired, that the band Widthhe asmall per-- centage of the meanrfrequency. ofthepass band.,

Another o bject of` the invention is. toy provide an improved band pass filter. having. fewer. come ponents thanconventionalv filters and providing.. aband'vwidth whichisnomore than vepercentof the mean frequency of the-passband.`

A further object. of` ther inventionl isto pro.-` Vide an improved band passlter includingIk an electro-mechanical resonator as: one of;the:cir cuit elements inthe, shunt armzof anequivalent Mlderived lter.

In accordance. with. thel inventions there is; provided an equivalentv M-derived.lterfimwhich the inductive components of, the,s,eriesy arms; aren closely coupled. inductively so .that theY resultingC mutual inductance forms a circuit element, of

the shunt, arm, Alsorthere is included anxelectrosmechanical vibratory element,` such, as amagnetostrictive. device` mounted. for excitation by the mutually. coupled inductive componentsV to form another circuit. elementy of theshuntarm;v In accordance with. a further, feature of.- the present invention, the, series., arms may include another pair of. inductively# coupled inductive. components so connected that.. the mutual. in-vdrawingi..

In.7 the drawings.

Fienre. 1f .instrateay an conventional; M-deniyedl. ltery which istheequivalent ofg the-.variousI modi. eations ofi the.: band. passI filters in`^ accordance.: with.. the presentimfentioru,Y

Eigure; 2,A is. aw curve depicting the response ohareotenistief off the.1 conventional M-derisred'. filter of-EPignre.11;

Figuresg 3;-, and;l 4;- respetatively;y illustrate.4 oney form ofi aha.rlduoassiilterl in accordance; with this invention'. and. its response; characteristic;

Figures. and Grespectively. illustrate; anotherr` form of a.y band pass iilter in, accordance;withthis, invention and.. its response.. chanacteristic;` and.

Figures 7 and 8 respectveln show a; third.-. modification 11i-abend nass inter. inaccordenoe with.. this^`v irlvention.k and.: its. response-A chene aoterstim Referring now toEigure. 1 of the drawingla;

conventional. band nass filter iscoupledv between.y input terminals, H; and output. terminals |12: The filter. includes, a. series resistor la and; two; series.. arm. meshes. le, and; le! oomnrsinei resneo tively-` av.. capacitor I6,. ai., coil or.: indnctor,V H, a; oolf or` inducter le; and` a. capacitor |19: "Ehe: lter. also, includestwo. shuntv arm: meshes; 2J.. and, 2.2; Connected. imseriesl between: the junction point. of, the, series* arm-Ameshes, l 4 and l5;- andthe grounded, inputgand outputterminals, |.I- andi I2., The., shunt alim.. mesh 2J comprises ai series` resonant circuit including a coil or inductonLigand..v a capacitor- 4L Theothen shunt arml mesh 22 comprises. a. parallel-Y resonant; circuit: including., ac`oi;or inductor, 25 and aapacitor 2,6. Final-- lyl the band; pass lter. includesE a.. resiston 251.r Connected.. in.y shunt. across. the` output.. tera Initials. l2?.

The. lter meshes., Iii..A l5, 21. and. 22. armena-f retelytuned to..thacenterrreouencmor therese:

band. At a frequency lower than the center frequency the shunt arm mesh 2| acts like a capacitance and the shunt arm mesh 22 acts like an inductance so that, at this lower frequency, there is provided a series resonant shunt path to ground from the junction point between the series arm meshes I4 and I5. The frequency at which this shunt circuit is eiective determines the lower limiting or cutoff frequency of the l'llter. At a frequency higher than the center frequency of the pass band, the mesh 2| acts like an inductance and mesh 22 acts like a capacitance so that, at this higher frequency `a similar series resonant shunt circuit to ground is provided from the junction point between the series arm meshes I4 and I5. The frequency at which this shunt circuit is effective in this manner determines the higher limiting or cutoff frequency of the lter.

In Figure 2, the curve 28 graphically represents a typical response characteristic of the conventional band pass lter of Figure 1 which, as will be pointed out, has practical undesired limitations. It is noted that this curve has three peaks 29, 30 and 3I of substantially equal amplitudes and that the skirts of the curve are reasonably steep. In order to construct a band pass lter having even as good a response curve as this, however, it is necessary that the shunt arm meshes 2l and 22 have Qs several times greater than the Qs of the series arm meshes I4 and I5. It has been found to be extremely dicult to obtain coils for use as the coils 23 and 25 which will have sufciently high Qs for use in a filter of this character when the ratio of the mean or center frequency of the pass band to the overall band width is as great as 20, for example. As a matter of fact, the curve 28 of Figure 2 represents the optimum performance that may be expected from a conventional band pass filter such as that shown in Figure 1. Where the center frequency such as represented by the peak 30 is of the order of 500 kilocycles, the band width represented by the skirts of the curve 28 is approximately 20 percent or more of the center frequency. For many purposes, it is desirable to have a band width which is no greater than 5 per cent of the mean frequency. It will be appreciated that such performance is not easily realizable by use of a conventional filter such as that shown in Figure l.

Having reference now to Figure 3, there is shown one form of band pass filter in accordance with the present invention. In this case, the coils i1 and I8 of the series arm meshes I4 and I 5 respectively are mutually coupled in quite close relationship to form in effect a transformer, the core of which is an electro-mechanical vibratory element, such as a magnetostrictive bar 32. Therefore, the bar 32 is directly inductively coupled to the coils I'I and I8. Preferably, this bar is formed of a good magnetostrictive material such as magnesium and zinc ferrite. The dimensions of this bar preferably are so chosen that it will resonate substantially at the center frequency of the band of frequencies to be passed by the filter.

It will be seen that the band pass filter in accordance with the invention eliminates the shunt arm coils 23 and 25 and also the capacitor 2B of the conventional lter shown in Figure l. It also requires only the addition of the magnetostrictive bar 32. In operation, the inductance corresponding to that represented by the coil 23 of Figure 1 isformed by the mutual inductance 4 between the coils I1 and I8 of Fig. 3. The shunt arm rejector mesh 22 of Figure 1 is replaced by the mechanical resonance of the magnetostrictive bar 32. Since the inductance which is equivalent to that contributed by the coil 23 of Figure l is the mutual inductance between the coils I1 and I8 of Figure 3, it is seen that this mutual inductance constitutes a low loss circuit element or one having a relatively high Q. Also, the magnetostrictive bar 32 has a relatively high Q and,

since both of these elements are included in the shunt arm meshes of the filter, it is seen that they meet the requirements of such a circuit in a more satisfactory manner than the shunt arm meshes ZI and 22 of Figure l.

The curve 33 of Figure 4 is a typical response curve of the form of the invention shown in Figure 3. It will be seen that this also is a curve having a relatively sharp cutoff at both of the limiting frequencies of the pass band. It has been found that, when using a band pass filter such as that disclosed in Figure 3, the band width is approximately 5 percent of the mean or center frequency.

Another form of the invention is shown in Figure 5. This embodiment is somewhat similar to the form shown in Figure 3 with the exceptions that the resistors I3 and 2l of Figure 3 are not included and the capacitor I9, which together with the coil I8 forms one of the series arm meshes is, in this instance, connected in shunt across the output terminals I2. The shunt arm circuit of the lter functions in a manner substantially similar to that of the shunt arm circuit of Fig-ure 3.

Figure 6 shows a response curve 34 for the form of the invention shown in Figure 5. It will be noted that the band width of the filter is approximately 2 percent of the mean frequency which may be somewhat narrower than desired for many purposes. Also, it will be observed from the curve 34 that this form of band pass filter does not effect complete attenuation of all frequencies outside of the pass band. Nevervtheless, for many applications it does have distinct advantages over prior art types of band pass filters, such as that represented in Figure 1.

Still another form of the invention is shown in Figure 7. This modication is of somewhat the same general form as that shown in Figure 5. In the embodiment of Figure 5 it may be found difiicult to obtain tight enough coupling to the magnetostrictive bar Without having the mutual inductance between the coils Il and I 3 larger than desired. In the embodiment of Figure *l this is taken care of by the provision of an additional transformer comprising coils 35 and 36 connected in series respectively with the coils II and I8. In this case, however, the coupling between the coils 35 and 36 is reversed to that of the coils I'I and I8. This allows the mutual inductance (corresponding to the inductance of the coil 23 of Figure l) to be reduced to the desired value without affecting the value of shunt arm mesh corresponding to the elements 2E and 26 of Figure l.

In Figure 7 the inductance corresponding to the inductance of the coil 23 of Figure l, is the combined mutual inductance of the coils il and IS and the mutual inductance of the coils 35 and 36. Thus, by reversing the coupling of the coils 35 and 36 as shown, the mutual inductance thereof is of opposite sign to the mutual inductance of the coils I'I and I8. Also, the inductance oi' the windings 35 and 35 was made somewhat less te@ hidaeteeca "tie teils the @01.1.5 3.5 au a, h

J L .v in Qi tente-.1mi andi-twas made su1 Y. `cientlylessl than the mutual inductance of the coils ll and I8 so that the net effective inductance; ofF chiav portion of the shunt c-ircuit, whichisj equivalent to the coil E@ of Figure l, was made sufficiently less thanin the forni o f the invention shown in Figure 5 so asa to provide the desired eiective value-"of inductance.

Figure 8 illustrates by means of a curve, g; the response characteristic ofthe form of the invention showinl in Figure 7-. From this curve itv will be seen thatit qff the. three peaked character, partcularlywbeat .s @911s Ramtha-shunt alim. circuit have. neatly bien, Q55. alge it has relatively sharp cutolfs at the limiting frequencies of the pass band. In this case, the pass band was somewhat less than 5 percent of the mean frequency. It also has been found that, even with coils of somewhat lower Qs, the three peaked response curve may not be realized in all cases but the lter will have substantially the same sharp cutoffs at the limiting frequencies.

It may be seen, therefore, from the foregoing description of several illustrative embodiments of the invention that there is provided a materially improved band pass filter Which, by the use of a magnetostrictive element as one of the shunt arm circuit elements and by suitably coupling the coils of the series arm meshes so as to take advantage of their mutual inductance, comprises a considerably smaller number of circuit elements than is required in conventional filters of this character. Furthermore, the coils used as the inductive elements may be suitably formed with reasonably high Qs so as to enable the filter in accordance with the invention to functionl in a much improved manner. Furthermore, such a lter provides a pass band at medium frequencies which is a much smaller percentage of the mean frequency than is possible of attainment by use of conventional band pass filters of the prior art.

In the several practical embodiments of the invention shown respectively in Figures 3, 5 and 7, the band pass filter was designed to operate at a mean frequency of approximately 480 kilocycles. Such fllters used as the magnetostrictive bar 3'2, one having the following dimensions: 0.271" x 0.1 x 0.0625. The coils Il and i8 were universal windings using three No. 4l Litz Wire, 3 wide, wound on a M3 diameter form, the coils having an outside diameter of They were placed approximately gaf" apart on the supporting form and the magnetostrictive bar was centered on the common axis of the coils. Capacitors i6 and I9 were 120 micromicrofarads each. The capacitor 24 was 2500 micromicrofarads.

It will be obvious to those skilled in the art that the present invention may be embodied in forms other than those specically disclosed herein for illustrative purposes. Accordingly, the scope of the present invention is to be determined by reference to the appended claims.

What is claimed is:

1. A band pass lter having a pair of input terminals and a pair of output terminals, comprising a pair of serially connected inductors having the terminals thereof coupled respectively to corresponding ones of said input and output terminals, a resonating capacitor connected to each of said inductors, said capacitors being coupled raaeairely.- u una are. @una incise-.tilts beine didi 0i r dine 1.1.'. of. meesters-i. .,f; Said. arid-ideaal. induce tors being connected in reverse polarity to the corresponding one of said rst mentioned pair of inductors, both of said additional inductors having lower values of inductance than said first mentioned inductors and also being coupled so that the resulting mutual inductance is less than and is of opposite sign to the mutual inductance resulting from the coupling of said rst mentioned pair of inductances.

2. A band pass filter, comprising two series arms and a shunt arm symmetrically connected to said series arms, each of said series arms comprising a mesh which includes rst and second pairs of inductors and which is resonant at the center frequency of said pass band, said first pair of inductors being coupled so that the resulting mutual inductance is effective in said shunt arm, said second pair of inductors having lower values of inductance than said first inductors and also being coupled more closely than, and in a reverse sense to the coupling of said first pair of inductors, so that the resulting mutual inductance also is effective in said shunt arm in opposition to the mutual inductance of said first pair of inductors, whereby to produce a net effective inductance in said shunt arm of a lower value than either of the mutual inductances produced by the respective couplings of said first and second pairs of inductors, a capacitor included in said shunt arm and forming with said net effective mutual inductance a mesh which is resonant at said center frequency, and an electro-mechanical vibratory element mounted for excitation by one pair of said inductors and forming another mesh in said shunt arm which is resonant at said center frequency.

3. A band pass filter as defined in claim 2, in which all of said inductors are connected in series with one of said second inductors having a reverse polarity, and said electro-mechanical vibratory element being mounted for excitation by said first pair of inductors.

4. A band pass filter as defined in claim 3, in which both of said second pair of inductors are connected directly together and the inductors of said rst pair are connected respectively to corresponding ones of said second pair of inductors, said capacitor being connected to the junction point between said second pair of inductors.

5. A band pass lter having input and output terminals, comprising two pairs of inductors connected in series with one of the inductors of one of said pairs being connected in opposite polarity to the other inductors and said series connection of said inductors being coupled between corresponding ones of said input and output terminals, each pair of said inductors being mutually coupled so that the resulting mutual inductance is eiTective in a shunt arm of said lter, the two pairs of inductors being related to one another ininductance Values in such a manner that the two resulting mutual inductances are of different values so that by reason of the reverse connection of one of said inductors there is produced a net effective mutual inductance in said shunt arm having a value less than that of either of said mutual inductances, a capacitor connected between the junction point of said serially connected inductors and the opposite ones of said input and output terminals forming with said net effective mutual inductance a shunt arm mesh which is resonant at the center frequency of said pass band,

References Cited 1n the le 0f this patent UNITED STATES PATENTS Number Name Date 1,624,665 Johnson et al Apr. 12, 1927 2,115,818 Lakatos May 3, 1938 2,166,359 Lakatos July 18, 1939 2,435,487 Adler Feb. 3, 1948 OTHER REFERENCES Book Transmission Networks and Wave Filters by T. E. Shea, Van Nostrand C'o., Inc., 1929. 

